This invention relates to polymer emulsions, and more particularly to the isolation of polymers therefrom by coagulation.
Many addition polymers are routinely prepared in the form of an aqueous emulsion or latex, containing polymer particles of a size typically in the range of 1-500 nm suspended in water. It is necessary to isolate said polymer from the emulsion for use. Isolation by coagulation, in which the polymer forms aggregated particles of a size larger than that of the emulsion particles, is usually most convenient.
U.S. Pat. Nos. 4,299,952 and 4,668,738 describe a shear coagulation method, in which a polymer latex is subjected to mechanical shear to form a paste. The paste is then treated with steam under pressure with further shearing, to raise the latex temperature to about 40-90.degree. C. and thereby achieve coagulation. It is usually found, however, that the polymer particle size in the paste is essentially the same as in the latex and that an additional isolation step is necessary to convert the polymer into crumbs which can be ground into a powder.
Other methods of isolation involve a step of chemical treatment of the emulsion, which is normally prepared and exists in alkaline form. Treatment reagents include acidic materials such as gaseous carbon dioxide and sulfuric acid as disclosed in U.S. Pat. Nos. 4,623,678 and 5,698,666 respectively; asphalt as disclosed in U.S. Pat. No. 5,025,044; and polyvalent metal salts such as zinc chloride, magnesium chloride or calcium chloride.
The chemical treatment methods have the disadvantage that various chemicals, such as the coagulating chemicals themselves and surfactants employed in formation of the emulsion, remain in the isolated polymer and can cause problems during processing and/or compounding. Asphalt is useful as a treatment agent only when a blend of the polymer and asphalt is desired, which is often not the case.
It remains of interest, therefore, to develop methods for polymer isolation from emulsion which produce large, easily processed polymer particles of convenient size for use without the necessity of multiple treatment steps during or after coagulation.